1. Field of the Invention
This invention relates to ultrasonic flowmeters and more particularly to axial type flowmeters in which the rate of fluid flow is measured by transmitting sonic energy through a linear flow path.
2. Description of the Prior Art
A well-known means of measuring fluid flow is by the use of sonic energy. By transmitting sonic energy through a moving fluid stream, the rate of movement of the stream can be detected. Various arrangements are practiced for transmitting the sonic energy through the fluid stream. U.S. Pat. No. 2,991,650 shows various configurations of sonic flowmeters. A common method is to affix sonic transducers on opposite sides of a conduit and with one of the transducers being displaced downstream from the other. Sonic energy is thereby transmitted diagonally across the fluid stream and at an angle through the longitudinal axis of the fluid stream. This system has advantages of simplicity of installation; however, difficulty in obtaining a high degree of accuracy is experienced.
In order to increase the accuracy of ultrasonic flowmeters, a commonly employed system is to transmit the sonic energy from the first to a second transducer longitudinally along a linear flow path. One method is that the transducers are positioned centrally within an enlarged diameter conduit with the fluid flow passing around the first transducer, through the linear flow path, and around the second transducer. This system, which is described in U.S. Pat. No. 3,817,098, has disadvantages in that it requires the transducers to be positioned within the flow stream, making it difficult to replace and repair them. In addition, the transducers disturb the flow that is to be measured and are thereby a source of turbulence. Electrical conductors to the transducers must be brought out the sidewall of the flowmeter conduit.
Another commonly used method of employing linear flow paths in ultrasonic flowmeters is the use of the U-tube design illustrated in U.S. Pat. No. 2,991,650 and in an alternate arrangement in U.S. Pat. No. 3,575,049. In this design flow is diverted at the inlet through an angular bend, the flow then passing through an axial flow path to a second angular outlet. Transducers can then be affixed at each end of the flow path exteriorly of the bends. This system has the advantage that the transducers are removable without entering the flow stream, and connectors to the transducers do not have to pass through the flow stream conduit. A disadvantage, however, is that the turns required by the fluid stream introduces turbulence in the fluid flow path. This turbulence affects the speed of sound transmission in the fluid and causes non-linearity and non-repeatability in the sonic flow measurements. To overcome this problem, a standard technique has been to increase the length of the axial flow path, and many manufacturers provide flowmeters in which the length is fifteen or more times the diameter of the axial flow path. While the longer length of the flow path improves the performance of the axial type flowmeter, it has disadvantages within itself in that it makes the application of flowmeters more difficult; that is, in many piping arrangements there is not sufficient room to install a flowmeter which requires a long axial length of the flow path. Further, the use of long axial flow paths, such as fifteen times the diameter of the axial conduit, does not, within itself, serve to alleviate all of the turbulence. For this reason, even axial ultrasonic flowmeters having relatively long flow paths are not as accurate as is frequently desired.
It is an object of the present invention to overcome some of these problems and difficulties in the prior art of axial-type ultrasonic flowmeters by providing arrangements wherein the linearity and repeatability of the flowmeters are enhanced.